To begin, then, the reasons for using transmission line structures are many and well known. As higher and higher frequencies are employed it is also increasingly likely that increasing degrees of integration are used to realize the circuitry. It is not, however, the case that this is always accomplished within the confines of a single piece of semiconductor material (that is, within one Integrated Circuit, or IC); it remains the case that the xe2x80x9chybridxe2x80x9d circuit consisting of a substrate with various thick film structures thereon that are interconnected with a plurality of ICs is a desirable technique. So it is that we find high frequency hybrids that include transmission line structures fabricated upon the substrate thereof; it becomes a preferred way of conveying signals from one IC on the hybrid to another. We are particularly interested in the case when the transmission line is of the encapsulated microstrip type described in the incorporated Patent. By the term xe2x80x9cencapsulatedxe2x80x9d that Patent means that the transmission line, which in their example is what would otherwise be called a microstrip, is fully shielded, with a ground completely surrounding the center conductor. It is not exactly what we would ordinarily term a xe2x80x9ccoaxialxe2x80x9d transmission line, since its cross section does not exhibit symmetry about an axis; it has a line and a rectangular trapezoid for a cross section instead of a fat point and surrounding circle. Nevertheless, we shall find it appropriate and convenient to call it (the xe2x80x98encapsulatedxe2x80x99 transmission line of the ""730 B1 Patent) a xe2x80x98quasi-coaxialxe2x80x99 transmission line, which, it should be noted, is pretty small (perhaps 0.050xe2x80x3 wide by 0.010xe2x80x3 or 0.015xe2x80x3 high, which makes the otherwise diminutive 0.100xe2x80x3 diameter RG 174/U seem large in comparison).
Sometimes the signals carried by these quasi-coaxial transmission lines must enter or leave the hybrid substrate, and this almost certainly means that some sort of coaxial connector of the controlled characteristic impedance variety is required.
There are xe2x80x9cright angle connectorsxe2x80x9d that are intended for similar service on printed circuit boards. They are generally easy to mount, and they afford fairly good shielding and good mechanical stability. But maintaining a truly controlled characteristic impedance (say, 50 xcexa9) at multi-gigahertz operation is not easy, and some right angle connectors can exhibit undesirable discontinuities at certain frequencies. Their xe2x80x9cstraight throughxe2x80x9d or xe2x80x9cend launchxe2x80x9d brethren do much better in this regard, but even they are not, as they come out of their package, a complete solution for attaching to the quasi-coaxial transmission line of a hybrid circuit, as described above and in the incorporated Patent to Dove, Casey and Blume. Part of the difficulty is that any of the standard sizes (say, of the SMA variety) may be of an awkward form factor to attach to such a small transmission line, and the substrate at hand may be significantly thinner than the thinnest standard printed circuit board. This can give rise to difficulties in keeping the characteristic impedance uniform during the transition from the connector to the transmission line. In addition, and this is by itself a significant issue, the straight through connectors offer no facility for maintaining the shielding around the entire center conductor during the actual transition. This lack of shielding allows for much potential mischief, including losses owing to radiation, interference in other nearby circuits caused by that radiation, and coupling of other unwanted signals into the unshielded transition to create interference.
It is not at present economically practical to develop a complete new connect or just to solve these problems. Instead, it is preferable to find a way to employ an otherwise suitable and readily available existing connector. What to do?
A solution to the problem of connecting a merchant straight through coaxial RF connector to an quasi-coaxial transmission line formed on the substrate of a hybrid is to, if necessary, gradually increase the height of the center conductor of the quasi-coaxial transmission line by increasing the thickness of underlying deposited dielectric until the center conductor of the transmission line matches the position of the center conductor of the connector, which two may then be joined with solder or conductive adhesive. One style of coaxial RF connector of interest has four prongs disposed in a rectangle around the center conductor on the permanent and non-threaded side. Two of the prongs define a long side of a rectangle and may be soldered or otherwise attached to the substrate with conductive adhesive, and the two prongs that define the long side closest to the center conductor can support a small cover that physically bridges and electrically shields the gap between the connector and the end of the quasi-coaxial transmission line. The cover provides complete shielding and assists in minimizing the discontinuity in characteristic impedance caused by the transition between the connector and the quasi-coaxial transmission line.